For wood screws, you can get unplated screws in different sizes as noted by Wayne at Blacksmith Bolt and Rivet Supply. Make a countersink plate out of spring steel or something tough that will cover sizes @ #4 to #10 or so. Take the unplated screws and beat the heads down into the countersink of appropriate size until you close up the machine cut slot. Now you can shape the head and cut the slot by hand for a more authentic look.
They sell long blanks or long rivet blanks that are awesome for making tang and lock bolts but *warning* they are EXTREMELY soft and easily bent while working and threading. Once they're cleaned up, shaped and threaded as needed, they should be casehardened.